Coaxial heat transfer tube suitable for a fluidized bed boiler and a method for manufacturing same

Abstract

A coaxial heat transfer tube (100), comprising a first inner heat transfer tube (110) having straight parts (111, 113, 115, 117) and curved parts (112, 114, 116), a first outer heat transfer tube (120) having straight parts (121, 123, 125, 127) and curved parts (122, 124, 126). The first outer heat transfer tube (120) radially surrounds the first inner heat transfer tube (110) at least within the first primary straight part (121) and the first primary curved part (122). Thermally insulating material (530) has been provided in between a curved part (122) of the first outer heat transfer tube (120) and a curved part (112) of the first inner heat transfer tube (110). Neither straight part (121) nor a curved part (122) of the first outer heat transfer tube (120) comprises a longitudinal seam. A method for making a coaxial heat transfer tube (100).

The invention claimed is: 
     
       1. A method for manufacturing a coaxial heat transfer tube, the method comprising the steps of:
 arranging an inner heat transfer tube and an outer heat transfer tube, 
 inserting at least a part of the inner heat transfer tube into the outer tube such that the inner heat transfer tube extends at least through the outer heat transfer tube to form a straight coaxial heat transfer tube, 
 arranging thermally insulating material in between the inner heat transfer tube and the outer heat transfer tube, 
 hardening the thermally insulating material at a temperature of 100° C. to 400° C., 
 after hardening the thermally insulating material, bending the straight coaxial heat transfer tube to form a first primary curved part to the coaxial heat transfer tube, and 
 before said bending, forming first protrusions on at least one of an inner surface of the outer heat transfer tube or an outer surface of the inner heat transfer tube, 
 wherein, after said hardening, a thermal conductivity of the thermally insulating material is from 1 W/mK to 10 W/mK at a temperature of 20° C. 
 
     
     
       2. The method of  claim 1 , wherein:
 the forming of the first protrusions occurs by punching blind holes onto an outer surface of the outer heat transfer tube. 
 
     
     
       3. The method of  claim 2 , wherein the blind holes are punched only to such parts of the tube that will remain straight after said bending. 
     
     
       4. The method of  claim 1 , wherein:
 before said hardening, the thermally insulating material is injected in between the inner and outer heat transfer tube; and 
 after said hardening:
 a Young's modulus of the thermally insulating material is at least 1 GPa or at least 5 GPa at a temperature of 20° C., and 
 the thermally insulating material is heat resistant at least up to 1000° C. 
 
 
     
     
       5. The method of  claim 1 , comprising the step of bending the straight coaxial heat transfer tube to form a first primary curved part to the coaxial heat transfer tube at such a temperature that the temperature of the inner heat transfer tube and the outer heat transfer tube is below 300° C. 
     
     
       6. The method of  claim 1 , comprising the step of centering the inner heat transfer tube and the outer heat transfer tube relative to each other by using the first protrusions.